Microwave power rectifier with multipactor discharge



July 21, 1970 M. P. FORRER MICROWAVE POWER RECTIFIER WITH MULTIPACTOR DISCHARGE Filed Dec. 17, 1964 IO 20 I'm I I E1 INVENTOR. MAX R FORRER ATTORNEY- United States Patent 3,521,146 MICROWAVE POWER RECTIFIER WITH MULTIPACTOR DISCHARGE Max P. Forrer, Neuchatel, Switzerland, assignor to General Electric Company, a corporation of New York Filed Dec. 17, 1964, Ser. No. 419,080 Int. Cl. H02m 7/00; H01j 43/00 US. Cl. 321--8 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to conversion of high frequency electromagnetic energy into direct current power. More particularly the invention relates to a microwave power rectifier utilizing a multipactor discharge.

While devices such as the semiconductor diode are known that can be used as eflicient microwave rectifiers, the power-handling capability of most such devices is limited.

It is the object of the present invention to provide a high frequency electromagnetic energy converter capable of rectifying high levels of power as may be used, for example, in microwave beam power transmission systems.

This and other objects are achieved according to the invention by the use of a multipactor cell as a power conversion device.

The multipactor discharge or multipactor effect is now well-known and is defined, for example, in the Encyclopedia Dictionary of Physics, vol. 4, pp. 475 and 476, Dergamon Press, 1961.

Briefly, a multipactor dicharge consists of an electron cloud in a vacuum which is produced by secondary emission from the surfaces of a pair of opposed spaced electrodes when the electrons are driven back and forth across the gap between the electrodes in synchronism with an applied high frequency electric field. Such a discharge may sustain itself if the secondary emission ratio is greater than unity and if the frequency and energy level of the electric field and the spacing of electrodes are such that synchronous motion is possible. Thus the frequency and energy level of the applied electric field and the spacing of the electrodes are chosen so that the time of transit of the electrons between the electrodes is equal to an odd multiple of a half-period of the high frequency wavefor rectifier use the transit should occur in one-half period.

In accordance with a first embodiment of the invention, the two opposing surfaces of a pair of spaced electrodes forming a multipactor gap are formed of dissimilar materials, one having a high secondary emission ratio and the other having a lower secondary emission ratio. There 3,521,146 Patented July 21, 1970 "ice is thus a time-average or net electron flow from the high emitter to the low emitter, this net electron flow constituting a direct current. The two electrodes are electrically insulated from one another so that the electrons collected by the low emitter may be applied to a direct current utilization circuit.

In accordance with'the preferred form-of a second embodiment of the invention, the two opposing surfaces of a pair of spaced multipactor electrodes froming a multipactor gap may be formed of similar material characterized by a high secondary emission ratio. Both electrodes are perforated or apertured so that a portion of the incident electrons from the electron cloud of the multipactor discharge passes through these perforations. Electrically isolated collector electrodes are positioned behind the multipactor electrodes to collect the electrons which pass through the perforation of the multipactor electrodes. The electrons thus collected constitute a direct current which may be applied to a direct current load. The rectifier of this second embodiment provides a full-wave rectification.

The invention is described more specifically hereinafter with reference to the accompanying drawing wherein.

FIG. 1 is an elevational view, in cross section, of the first embodiment of the invention;

FIG. 2 is an elevational view, in cross section, of the second embodiment of the invention; and

FIG. 3 is a schematic illustration of the embodiment of FIG. 2.

A cavity of the coaxial or re-entrant type is a convenient device for coupling microwave power to a multipactor gap since the multipactor electrodes can be placed in the center ofthe cavity in a region of high and substantially uniform electric fields.

The multipactor rectifier structure of FIG. 1 comprises a cavity 10 formed of a cylindrical conductive wall 11 and conductive plane upper and lower walls 12 and 13. A closed-end cylindrical conductive member 14 is fitted in a central aperture of the lower wall 13 to form a cylindrical re-entrant portion of the cavity. A similar closedend cylindrical conductive member 15 is positioned above the member 14 with the adjacent closed ends of members 14 and 15 being in spaced relation. The member 15 is held in position by an insulating ring 20, formed of a material such as ceramic or glass, the ring 20 thus also serving to electrically insulate the member 15 from the cavity and to contain a vacuum in a region 18 between the closed ends of the members 14 and 15 while permitting passage of the electromagnetic energy into the region 18.

The closed ends of the members 14 and 15 constitute the multipactor discharge electrodes and they are spaced to form a gap in the region 18 appropriate in relation to the frequency and energy level of the applied microwave power to support a multipactor discharge therebetween. In the present example, suitable secondary emission surfaces are provided by forming on the opposing closed ends of members 14 and 15 suitable films or surface coatings 1'6 and 17.

In accordance with the first embodiment of the invention, the surface 16 is formed with a material, such as copper, having a relatively low secondary emission ratio n the Surface 1 is m dw t a ma e iatns has an.

alloy of silver' and magnesium, having a relatively high secondary emission ratio. (See, for example, V. K. Zworykin et al., Silver-Magnesium Alloy as a Secondary Electron Emitting Material, I. Appl. Phys., vol. 12, pp. 696- 698, September 1941.)

Thus, in the presence of a multipactor discharge between surfaces 16 and 17, a greater number of electrons will be emitted from the surface 17 because of its higher secondary emission ratio. There is therefore a net electro flow from the surface 17 to the surface 16, this net ele tron flow constituting a direct current. The electrons thu collected at the surface 16 may be applied to a direct cur rent utilization circuit illustrated in FIG. 1 as a resistor, 28. Resistor .28 has a pair of terminals 25 and 27 which are connected between the multipactor electrode surfaces 16 and 17 by means of a pair of leads 24 and 26.

The microwave power to be rectified is provided by a source 23, illustrated schematically, which isconnected to cavity through a coaxial coupler 22, the inner conductor of which is formed into a coupling loop within the cavity 10.

To prevent leakage of the electromagnetic energy from the cavity 10, the upper wall 12 is formed with acylindrical upstanding portion 29 disposed coaxially with and spaced from member 15. The portion 29 is approximately one-quarter wavelength in height at the operating frequency. Thus, the portion 29 and the adjacent portion of member function as an open-ended quarter-Wavelength choke or filter to prevent power leakage while maintaining the necessary D-C isolation of the member 15.

Thus, the multiplactor conversion device of FIG. 1 provides half-wave rectification of the applied microwave power through the agency of dissimilar multipactor electrode surfaces, the surface of lower secondary emission ratio collecting the net electron flow for application to a direct current utilization circuit.

An embodiment of the invention which provides fullwave rectification is illustrated in FIGS. 2 and 3, FIG. 3 beng a schematic diagram of the structure of FIG. 2.

The structure of FIGS. 2 and 3 comprises a cavity 30 formed of a cylindrical conductive wall 31 and upper and lower plane conductive Walls 32 and 33. Lower wall 33 is formed with a cylindrical re-entrant member 34 while upper wall 32 is formed with a similar re-entrant member 35.

A pair of disk-shaped, spaced conductive members 36 and 37 constitute multipactor electrodes, these electrodes "being supported by the inner ends of the re-entrant members 34 and 35. The opposing surfaces of multipactor electrodes 36 and 37 form a multipactor gap 38, the gap spacing being designed to support a multipactor discharge for the frequency and power level of the microwave power to be rectified. The opposing surfaces of multipactor electrodes 36 and 37 are preferably formed with material having a high secondary emission ratio such as a suitable alloy of silver and magnesium as previously mentioned.

In the embodiment of FIGS. 2 and 3, the multipactor electrodes 36 and 37 are gridded, or formed with perforations such as a perforation 39 in multipactor electrode 36. These perforations allow the escape of a portion of the electrons from the multipactor discharge, these electrons thus constituting the direct current output power of the device. The ratio of the area of the perforations to the remaining secondary emitting area of the surfaces of the multipactor electrodes is an important factor. On the one hand, the perforation area is made as large as possible for maximum conversion efiiciency. 0n the other hand, the secondary emitting area must remain large enough so that the net electron gain through secondary emission (less the number of escaping electrons) is still at least unity; otherwise, the multipactor discharge will not be maintained.

To collect the electrons which escape through the per electr 0des,-.40 and .41 areprovided, each of the collector electrodes 40 and 41 being positioned behind a respective one of the perforated multipactor electrodes 36 and 37 in the path of the escaping electrons. Collector electrode 40 is supported by an insulating ring 42, formed for ex ample of ceramic or glass, and a conductive disk 44. The ring 42 serves to electrically insulate the collector electrode 40 from the cavity 30, the ring 42 being fixed to the cavity 30 and the disk 44 with vacuum tight seals. The collector electrode 41 is similarly supported by an insulating ring 43 and a conductive disk 45.

The collector electrodes 40 and 41 may be electrically joined, as by a pair of leads 53 and 54, whereby a direct current load, illustrated as a resistor 55, maybe connected in a full-Wave rectifier configuration, a terminal 52 of resistor SS-being connected to lead 54 and a terminal 50 of resistor 55 being connected to a lead 51 through the walls of-cavity 30 to the/multipactor electrodes 36 and 37.

The microwave power to be converted is provided by a source 47, illustrated schematically, which is connected to cavity 30 through a coaxial coupler 46, the inner conductor of which is formed into a coupling loop inside the cavity 30. The coaxial coupler 46 is vacuum sealed by an insulating sleeve '48 so that the interior of cavity 30 may be evacuated to provide the required evacuated environment for the multipactor discharge.

Thus what has been described is a device for collecting a portion of the electrons from a multipactor discharge by which the efiicient direct conversion of high-level microwave power to direct current power may be achieved.

While the principles of the invention have been made clear in the illustrative embodiments, there will be obvious to those skilled in the art, many modifications in structure, arrangement, proportions, the elements, materials and components used in the practice of the invention, and otherwise, which are adapted for specific environments and operating requirements, without departing from these principles. The appended claims are therefore intended to cover and embrace any such modifications within the limits only of the true spirit and scope of the invention.

What is claimed is: 1. A full-wave rectifier for microwave energy, comprising: a pair of spaced multipactor electrodes adapted to support a multipactor discharge therebetween in response to said microwave energy, said multipactor electrodes being formed with perforations to pass a portion of the electrons directed theretoward by said multipactor discharge; a pair of collector electrodes one positioned behind each of said multipactor electrodes to intercept the electrons passed by said perforations in said multipactor electrodes; a direct current load circuit connected between said multipactor electrodes and said collector electrodes and means for coupling microwave energy of a frequency and energy level between said electrodes to provide a secondary emission discharge between opposing surfaces thereof as a result of the motion of secondary electrons in synchronism with the alternating field of the microwave energy.

2. An electromagnetic energy rectifier, comprising: a pair of opposed, spaced and perforated multipactor electrodes, the opposing surfaces of said eelctrodes comprising a material having a secondary emission ratio greater than unity for electromagnetic energy in the region between said opposing surfaces of predetermined frequency and energy level, said multipactor electrodes being contained in an evacuated cavity; means for coupling electromagnetic energy of said predetermined frequency and energy level to said region to provide a secondary emission discharge between said multipactor electrodes as a result of motion of secondary electrons produced at said surfaces in synchronism with the alternating field of said electromagnetic energy; and electron collection means electrically insulated from said multipactor electrodes and positioned to collect electrons of said secondary emission discharge which pass through the perforations of said multipactor electrodes.

3. A microwave power rectifier, comprising: a pair of opposed, spaced and perforated multipactor electrodes; a conductive cavity containing said multipactor electrodes in an evacuated environment; means for coupling said microwave power to said cavity for establishing a multipactor discharge :between said multipactor electrodes; and collection electrodes insulated from said multipactor electrodes and positioned to intercept electrons escaping said 10 References Cited UNITED STATES PATENTS 3,201,640 8/1965 Farnsworth. 3,278,865 10/1966 Forrer.

WILLIAM H. BEHA, JR., Primary Examiner U.S. Cl. X.|R. 

